This invention relates to video discs and more particularly to creating a center hole in such a disc that is concentric to a formed information storing spiral groove.
A typical method of forming a video disc suitable for reproducing information stored therein is described in a copending U.S. application in the name of Jon K. Clemens, Ser. No. 126,772, now U.S. Pat. No. 2,842,194, assigned to the same assignee as the present invention. According to the method described by Clemens, an aluminum disc is machined flat and a thin lacquer coating is formed on the machined surface. The lacquer formed on the surface of the disc is first strained through a series of micro-mesh filters and then poured over the surface of the disc while the disc is rotated at about 10 rpm. Disc rotation is continued until a smooth lacquer coating is formed on the metal surface when dried. After several days of drying, the lacquer surface is further machined flat in preparation for subsequent formation of a spiral, information-storing groove.
The lacquer-coated disc is then mounted for rotation about its apparent center on the turntable of a record cutting lathe and is rotated at a substantially constant velocity of about 15 rpm. Typically, a hole formed at about the center of the lacquer-coated disc is used as an aid to center the disc upon the turntable. A heated cutting stylus is positioned on the disc and a spiral groove is formed therein. Stylus heating is desirable to provide smooth cutting of the lacquer.
Signal information may be formed in the spiral groove of the disc by modulating the cutting stylus motion to form a signal-representative topography concurrently with the formation of the spiral groove. A similar method is utilized in the audio recording art and may be extended to video recording with the aid of a cutting stylus arrangement having a relatively high frequency response. Another method of forming an information-representative topography utilizes the beam of a scanning electron microscope to expose an electron beam sensitive material which is coated on a spiral grooved substrate. A system of the latter type is described in the above-mentioned patent of Jon K. Clemens.
After the spiral groove and topography have been formed in either the lacquer-coated disc or the disc coated with electron beam sensitive material, a metal replica is made thereof by methods known in the audio recording art. This metal replica, having groove and topography of opposite contour from that of the lacquer, may be used to stamp or emboss discs of, for example, thermoplastic material. The thermoplastic replica is then metalized to make the surface conducting and the metalization is thereafter coated with a dielectric. In playing back the recorded information, a metal-coated stylus is caused to ride in the dielectric-coated groove. This stylus, along with the metalization and dielectric, acts as a capacitor. Capacitance variations in the groove, which correspond to the recorded video information, are then detected electronically to recover the video information.
Video information typically is recorded on the disc in an image-line-sequential format requiring precise timing of each line with respect to others. A deviation in the linear velocity between the spiral groove and the signal pick-up stylus of the pressed or embossed record as compared with that existing during recording results in a non-linear presentation of the decoded signal information on a monitor, such as a television receiver. If the spiral information groove on the disc is not concentric with the center hole on the playback disc, the information extracted from the disc will have a recurrent or periodic velocity error. This velocity error manifests itself as frequency modulation of the extracted video information and causes undulations in the vertically disposed image components displayed on the video monitor. Deviations in concentricity between the spiral groove and center hole in the disc may result from errors which occur during the early stages of playback disc formation such as during the mastering process or in the centering of the stamper in the stamping press. In the mastering process, the lacquer-coated aluminum disc may be imperfectly centered on the turntable of the spiral-cutting lathe. Such a deviation in centering causes the formation of the spiral groove to be about an apparent center rather than the actual one. If the center hole of the lacquer-coated disc is utilized as a means for locating the center hole in a stamper, and thereby center holes in the embossed or pressed playback discs, then such a centering error in the lacquer-coated disc would be undesirably transferred to the playback discs.
A similar centering error may occur if the center of the stamper is located by, for example, rotating the stamper on a turntable and continuously repositioning the stamper on the turntable until a groove located near the outside of the stamper disc appears to remain stationary during rotation. This method of centering is not precisely accurate and may result in erroneous location of the center of the stamper and consequently error in the location of the center of the embossed or pressed disc.